1. Field of the Invention
This invention relates to a spread spectrum signal demodulator, and more particularly, to a spread spectrum signal demodulator which is used for cellular communication systems, indoor wireless communication systems, wireless LAN (local area network) systems, etc. and in which an interference removing ability is improved.
2. Description of the Related Art
In communication by means of a spread spectrum signal, modulation symbols to be transmitted are further modulated by high-speed spread codes (this is called as a secondary modulation), and information is transmitted as a signal having a spectrum width not less than an information bandwidth. On a receiving side, a receiving signal is inversely spread and modulated symbols are extracted. Here, the inverse spread is a process for convolutionally integrating a conjugate complex of the spread code allocated to one-modulation symbol and the received signal. A symbol correlation value obtained as the integrated result is a demodulation symbol, and in the conventional technique, the correlation value for one symbol is directly subject to extraction of transmission information as the demodulation symbol.
Modulation symbols are obtained by directly modulating transmission information or by coding the transmission information for error correction and modulating the transmission information. As a receiving method, there exist a method of directly extracting transmission information from demodulation symbols and a method of replicating an interference component from demodulation symbols in an interference station and subtracting the interference component from a receiving signal so as to extract objective transmission information.
There will be explained below an example of the latter receiving method, namely, the receiving method utilizing interference removal which is applicable regardless of existence/non-existence of a pilot symbol with reference to FIG. 10. A receiving signal 1 is input into symbol decorrelating means 11 and a subtracter 13. The symbol decorrelating means 11 obtain a correlation value for one symbol and outputs it to interference replicating means 12. The interference replicating means 12 respread the input correlation values according to timing of transmission symbols so as to create replications of respective spread spectrum signals included in the receiving signal. The subtracter 13 removes the replicated signals from the receiving signal 1, and a symbol decorrelating means 14 obtains a correlation value between the receiving signal from which the replicated signals have been removed and spread codes allocated to the respective spread spectrum signals included in the receiving signal. An information demodulating means 15 demodulates the correlation value so as to output the demodulated value as demodulation information.
Next, there will be explained below a receiving means which is capable only to a transmission system where a pilot symbol exists with reference to FIG. 11. Here, since the components shown by the same reference numerals as those in FIG. 10 have the same or equivalent functions, the description thereof is omitted. Channel estimating means 16 estimate channel states from pilot symbols decorrelated by the symbol decorrelating means 11, and symbol deciding means 17 temporarily decide symbols from correlation value for one symbol. The interference replicating means 12 multiply the temporarily decided results by the channel estimated values and they are replications of interference components.
The above-mentioned conventional receiving means utilizing interference removal have a problem that since individual demodulation symbols are decided from a correlation value for one symbol, a noise component included in the correlation value for one symbol deteriorates the quality of a replicated interference signal, and accuracy of the interference removal is lowered.
It is an object of the present invention to provide a spread spectrum signal demodulator in which a noise component included in respective demodulation symbols is reduced and an effect of interference removal is improved.
In order to accomplish the above object, the present invention is first implemented by comprising symbol decorrelating means for decorrelating symbol correlation values between spread codes allocated to spread spectrum signals and a receiving signal; a symbol deciding means for deciding a transmission symbol based on the symbol correlation values decorrelated by said symbol decorrelating means; and correlation modification means for modifying the symbol correlation values based on the symbol decided result decided by said symbol deciding means, wherein a noise component included in respective demodulation symbols is reduced.
The present invention is second implemented by comprising symbol decorrelating means for decorrelating symbol correlation values between spread codes allocated to spread spectrum signals and a receiving signal; a symbol with reliability deciding means for deciding a transmission symbol based on the symbol correlation values decorrelated by said symbol decorrelating means and outputting reliability of the symbol decided result; and correlation modification means for modifying the symbol correlation values based on the symbol decided result decided by said symbol with reliability deciding means.
According to the first and second characteristics of the present invention, while a pilot symbol free which is a characteristic of a conventional replicated signal removing means according to soft decision basis (to be applicable even into a system where known information such as a pilot symbol for estimating channel state is not transmitted) is being maintained, an interference removing ability can be improved. Moreover, even in the case where the present invention is applied to a transmission system where a pilot symbol exists, the interference removing ability can be improved similarly.